1. Field of the Invention
This invention relates to a method and apparatus for repairing defects in photomasks generally used in the manufacture of semiconductor microcircuits and other electronic components to transfer a circuit pattern onto a workpiece, and, in particular, to repairing transparent defects in the process of making an attenuated mask using a transfer layer step. The defects are in the form of pin holes or entire missing portions in the transfer layer mask pattern which defects are transferred to the attenuated mask and eventually to the electronic component substrate during use of the attenuated mask.
2. Description of Related Art
In the manufacture of circuit patterns on electronic components such as the manufacture of integrated circuits on semiconductor substrates, photomasks are used to transfer the desired circuit pattern onto the substrate workpiece. A typical conventional photomask comprises a patterned metal film such as chromium, nickel or aluminum in a thickness of about 1000 .ANG. deposited on a transparent base such as glass or quartz. The photomask is generally manufactured by depositing a thin layer of the metal on the surface of the transparent substrate, coating the film metal with a photoresist, exposing the desired pattern on the photoresist coating, developing the photoresist coating, and removing the metal from the developed areas of the film by etching then removing the remaining photoresist leaving a patterned metal film on the substrate.
The pattern contained in the photomask is reproduced onto the surface of the workpiece typically by placing the mask over the workpiece and irritating a radiation-sensitive resist material on the workpiece. The variety of radiation sources include visible light, ultraviolet light, x-ray radiation, electrons and ions. When illuminated by the radiation, the metal pattern on the photomask serves to selectively block portions of the radiation beam while allowing other portions to be transmitted through the non-metallized areas. In this procedure, complex geometry having very narrow line widths can be reproduced allowing an economical production of very large scale integrated circuits and other electronic devices.
The type of mask to which this invention is particularly directed is an attenuated mask which is built using a transfer layer of chrome that allows the chrome to act as a mask to the attenuated film under the chrome layer. The transfer layer of chrome is generally patterned as noted above with the difference being that the chrome will eventually be removed exposing a pattern of attenuated mask material under the chrome forming the desired attenuated mask circuit pattern.
To make an attenuated mask, an attenuation layer is formed on a substrate such as glass or quartz by coating a layer of an attenuated mask material such as molybdenum silicide (MoSi) on the substrate surface. A chrome transfer layer is then applied over the attenuated mask material coating and is patterned as discussed above to form the desired chrome pattern and ultimately the desired attenuated mask material pattern which is under the chrome pattern on the substrate surface.
If there are defects in the chrome pattern such as missing chrome these are termed transparent defects and can take the form of pinholes or entire missing portions of the chrome film. Such defects in the chrome are also transferred to the attenuated mask material and to the mask during the development step. This is a serious problem since matching of the phase and transmission properties of the attenuated mask material is currently impossible.
To eliminate the transfer of the transparent defects into the attenuated film, a repair is generated that is resistant to the etch of the final attenuated film such as a plasma etch. This is typically accomplished using a FIB patch made primarily of gold and carbon. This material is inert to the etch used to remove the attenuated mask material such as MoSi. Unfortunately, the patch is also inert when the chrome is to be etched so that when the chrome is removed from the mask, the gold and carbon patch material remains because of the inability of the chrome etch to attack the patch material. With such residual or remaining patches on the mask, the attenuated film properties are effected and cause printed defects on the wafer substrate when imaged using the attenuated mask.
A number of patents have issued in the area of repairing defective conventional chrome photomasks and include U.S. Pat. Nos. 4,340,654; 4,510,222; 4,636,403; 4,778,693; 4,874,632; 5,011,580; and U.S. Pat. No. 5,273,849. These patents discuss the various types of conventional photomasks and the different methods employed to repair transparent type defects on the photomasks and the disclosures of each of the above patents are hereby incorporated by reference.
The following description will be directed to the repair of attenuated phase shift masks which may be made using a variety of attenuated mask materials such as MoSi, carbon and silicon nitride but it will be appreciated by those skilled in the art that the invention applies to other masks where repair of transparent defects is needed.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method to repair transparent defects in the fabrication process for making attenuated masks which are used to make electronic components such as semiconductors.
It is another object of the present invention to provide an apparatus to repair transparent defects in the fabrication process for making attenuated photomasks used to make semiconductors and other electronic components.
A further object of the invention is to provide attenuated photomasks made in accordance with the method and apparatus of the invention.
Other advantages of the invention will be readily apparent from the following description.